Method for tensioning and positioning a fiber optic cable

ABSTRACT

A method for tensioning and positioning a fiber optic cable includes providing and securing a first portion of the fiber optic cable in a first support with a first clamp. A second portion of the fiber optic cable is then provided in a second support, and secured thereto with a second clamp. A cam contacting the second support is then rotated, thereby rotating the second support due to its weight and the weight of the second clamp. The rotation of the second support creates a gravity-assisted moment arm that uniformly and repeatably tensions and positions the fiber optic cable. After the fiber optic cable is uniformly tensioned and positioned, a refractive-index grating may be etched in the glass optical fiber portion of the cable. Once the grating is etched, the cable may be removed by reversing the method.

BACKGROUND OF THE INVENTION

A. Field of the Invention

The present invention relates generally to the communications field,and, more particularly to a fiber optic cable tensioning and positioningapparatus and method for tensioning and positioning a fiber optic cableusing the same.

B. Description of the Related Art

Along with the increasing prominence of the Internet has come thewide-ranging demand for increased communications capabilities, includingmore channels and greater bandwidth per channel. Optical media, such asfiber optic cables, promise an economical alternative to electricalconductors for high-bandwidth long-distance communications. A typicalfiber optic cable includes a silica core (glass optical fiber), a silicacladding, and a protective coating. The glass optical fibers of fiberoptic cables have very small diameters, which are susceptible toexternal influences such as mechanical stress and environmentalconditions. The index of refraction of the core is higher than the indexof refraction of the cladding to promote internal reflection of lightpropagating down the core.

An optical fiber diffraction grating can output light having a specificreflection wavelength upon reception of incident light. Owing to thisadvantage, a great deal of attention has recently been paid to theoptical fiber diffraction grating as an important optical part in awavelength division multiplex (WDM) optical transmission communicationsystem which multiplexes and transmits optical signals having differentwavelengths through one optical fiber.

The signal carrying ability of fiber optic cables is due in part to thecapability of producing long longitudinally-uniform optical fibers.However, longitudinal variations in index of refraction, e.g., thoseassociated with refractive-index gratings, can be included in the fiberoptic cables to affect throughgoing pulses in useful ways. Gratings canbe grouped into short-period, e.g., about 0.5 micron (μm), orlong-period, e.g., about 200μm, gratings. Short-period gratings canreflect incident light of a particular wavelength back on itself in thefiber. Long-period gratings can couple incident light of a particularwavelength into other co-propagating modes on the fiber. Some of theseother co-propagating modes may be lost, so the overall effect of thelong-period grating can be to selectively block certain wavelengths frompropagating efficiently through the fiber.

While there are many methods for establishing a diffraction (orrefractive-index) grating within a fiber, one method involves exposingphotosensitive glass optical fibers to patterned light, via lasers. Theindex of refraction of certain fiber-optic materials, such asgermanium-doped silica, is changed upon exposure to mid-ultra-violet(mid-UV) light, e.g., wavelengths between 190 nanometers (nm) and 270nm. The lasers are used to etch lines in the glass optical fiber that isexposed (the coating removed) in the fiber optic cable.

In order to precisely form a refractive-index grating within a fiber, itis preferable to apply a repeatable, uniform tension on the fiber opticcable. A uniform tension ensures that the grating period is consistentacross the grating length. A repeatable tension ensures grating periodconsistency from fiber to fiber. If different tensions are applied fromone fiber to the next, the fibers will relax by different amounts andthereby cause different spacings between grating lines. In other words,the fiber is somewhat elastic and will stretch when tension is appliedand relax when the tension is released. Thus, applying inconsistentamounts of tension to a series of fibers being etched will result in aninconsistent grating period. The grating period tolerance for opticalcommunications equipment is extremely demanding and will not admit suchinconsistencies.

Another preferable feature would be to have an apparatus that is able toprecisely position the fiber in a repeatable manner. Otherwise, thegrating laser beam(s) will need to be aligned for each etching whichslows down the manufacturing process and is quite inefficient.

Tensioning the fiber may also help reduce grating period inconsistenciesin another way. More specifically, if the fiber is allowed to sagbetween two points it will form a catenary curve. Projecting a planargrating pattern on a catenary curve may result in a change in gratingperiod across the grating length. Tensioning the fiber reduces or eveneliminates the curvature of the catenary and, thereby, improves thegrating period consistency. A repeatable tension force further improvesgrating period consistency from one fiber etching to the next.

Conventional fiber tensioning apparatuses must be finessed a technicianto tension the fiber optic cable. Thus, these apparatuses suffer fromthe potential for human error and fail to provide a repeatable, uniformtension in the fiber optic cable while etch lines are formed in theglass optical fiber. Even if a skilled technician accurately tensions aparticular fiber optic cable, it is virtually impossible for thetechnician to provide the same tension for a series of cables.

Thus, there is a need in the art to provide an apparatus and a methodfor accurately and consistently tensioning a fiber optic cable, as wellas uniformly tensioning a series of fiber optic cables that are to haveidentical refractive-index gratings.

SUMMARY OF THE INVENTION

The present invention solves the problems of the related art byproviding an apparatus and method for uniformly and consistentlytensioning and positioning a fiber optic cable that eliminates thepotential for human error by a technician. The apparatus and method ofthe present invention relies upon gravity to provide a uniform,repeatable tension to a fiber optic cable, as will be described morefully below. The apparatus and method is thus useful for uniformlytensioning a multitude of fiber optic cables that are to have identicalrefractive-index gratings.

In accordance with the purpose of the invention, as embodied and broadlydescribed herein, the invention comprises a method for tensioning andpositioning a fiber optic cable, including: securing a first portion ofthe fiber optic cable to a first support; securing a second portion ofthe fiber optic cable to a second support; and creating agravity-assisted moment arm with the second support to uniformly andrepeatably tension and position the fiber optic cable between the firstand second supports.

Further in accordance with the purpose of the invention, as embodied andbroadly described herein, the invention comprises a method for forming arefractive-index grating in a fiber optic cable, including: securing afirst portion of the fiber optic cable to a first support; securing asecond portion of the fiber optic cable to a second support; creating agravity-assisted moment arm with the second support to uniformly andrepeatably tension and position the fiber optic cable between the firstand second supports; and etching grating lines in the fiber optic cableafter the fiber optic cable has been uniformly and repeatably tensionedand positioned.

Still further in accordance with the purpose of the invention, asembodied and broadly described herein, the invention comprises a methodfor calibrating a fiber optic cable tensioning and positioning apparatushaving a first support and a second support rotatable relative to thefirst support, including: securing a first portion of the fiber opticcable to the first support; securing a second portion of the fiber opticcable to the second support; measuring a diffraction grating provided inthe untensioned fiber optic cable; creating a gravity-assisted momentarm with the second support to uniformly tension the fiber optic cablebetween the first and second supports; measuring the diffraction gratingprovided in the uniformly tensioned fiber optic cable; and comparing themeasured diffraction grating of the untensioned fiber optic cable to themeasured diffraction grating of the tensioned fiber optic cable tocalculate the tension applied to the fiber optic cable.

Further scope of applicability of the present invention will becomeapparent from the detailed description given hereinafter. However, itshould be understood that the detailed description and specificexamples, while indicating preferred embodiments of the invention, aregiven by way of illustration only, since various changes andmodifications within the spirit and scope of the invention will becomeapparent to those skilled in the art from this detailed description. Itis to be understood that both the foregoing general description and thefollowing detailed description are exemplary and explanatory only andare not restrictive of the invention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description given hereinbelow and the accompanying drawingswhich are given by way of illustration only, and thus are not limitativeof the present invention, and wherein:

FIG. 1 is perspective view of a fiber optic cable tensioning apparatusin accordance with an embodiment of the present invention;

FIG. 2 is a side elevational view of the fiber optic cable tensioningapparatus shown in FIG. 1, with fiber clamps in an open position forreceiving a fiber optic cable;

FIG. 3 is a right side elevational view of the apparatus shown in FIG.2;

FIG. 4 is a top plan view of the apparatus shown in FIGS. 2 and 3;

FIG. 5 is a side elevational view of the fiber optic cable tensioningapparatus shown in FIG. 1, with fiber clamps in a closed position andthe fiber optic cable in tension;

FIG. 6 is a right side elevational view of the apparatus shown in FIG.5;

FIG. 7 is a top plan view of the apparatus shown in FIGS. 6 and 7;

FIG. 8 is an enlarged fragmental side elevational view showing the fiberoptic cable etched with refractive-index grating lines; and

FIG. 9 is flow chart showing a method of tensioning a fiber optic cablein accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following detailed description of the invention refers to theaccompanying drawings. The same reference numbers in different drawingsidentify the same or similar elements. Also, the following detaileddescription does not limit the invention. Instead, the scope of theinvention is defined by the appended claims and equivalents thereof.

Referring now specifically to the drawings, a fiber optic cabletensioning and positioning apparatus according to the present inventionis illustrated in FIG. 1, and shown generally as reference numeral 10.The tensioning apparatus 10 includes a horizontal base 12 integrallyconnected to a vertical support wall 14. A plurality of feet 16 supportsbase 12. A first fiber optic cable support 18 is rigidly affixed to aportion of vertical support wall 14. A first fiber optic cable clamp 20connects to a portion of first fiber optic cable support 18.

A second fiber optic cable support 22 pivotally attaches to a portion ofvertical support wall 14 and is spaced from first fiber optic cablesupport 18. A second fiber optic cable clamp 24 connects to a portion ofsecond fiber optic cable support 22. Apparatus 10 further includes aknob 26 connected to a cam 28. Both knob 26 and cam 28 pivotally attachto vertical support wall 14, wherein rotation of knob 26 causes cam 28to rotate. Apparatus 10 also includes an alignment mechanism 60pivotally attached to vertical support wall 14.

FIGS. 2-4 show the tensioning apparatus 10 as a fiber optic cable 100 isinitially provided into apparatus 10, and before fiber optic cable 100is tensioned. As shown in FIG. 2, first fiber optic cable support 18includes a grooved curved portion 30 (grooves best shown in FIG. 3) forreceiving a first portion of fiber optic cable 100. Preferably, theradius of curvature of grooved curved portion 30 is greater than theminimum bend radius of cable 100. First fiber optic cable support 18 isrigidly attached to vertical support wall 14 via a plurality ofconventional attaching means 32 (e.g., screws, rivets, bolts, etc.).

Second fiber optic cable support 22 includes a grooved curved portion 34(grooves best shown in FIG. 4) for receiving a second portion of fiberoptic cable 100. Preferably, the radius of curvature of grooved curvedportion 34 is also greater than the minimum bend radius of cable 100.Second fiber optic cable support 22 pivotally attaches to verticalsupport wall 14 via a pivot pin 36 extending through vertical supportwall 14. Preferably, a low-friction bearing (not shown) is providedwithin pivot pin 36 to ensure pivot pin 36 freely pivots.

Similarly, knob 26 and cam 28 pivotally attach to vertical support wall14 via a pivot pin 29 provided through vertical support wall 14. Knob 26is concentrically mounted on pivot pin 29, whereas cam 28 is notcentered on pivot pin 29. Second fiber optic cable support 22 furtherincludes a leg portion 38 that is forced downward by cam 28 when cam 28is in the position shown in FIG. 2. This permits a technician to loadfiber optic cable 100, without tensioning cable 100. As further shown inFIG. 2, the exposed glass optical fiber 102 of fiber optic cable 100 islocated between first and second supports 20, 22, and ideally centeredbetween supports 20, 22.

As best shown in FIGS. 5-7, alignment mechanism 60 includes a baseportion 62 pivotally attached to vertical support wall 14, via pivotpins 64. An arm portion 66 extends away from and is integral with baseportion 62. A head portion 68 is integral with and connects to armportion 66. Head portion 68 has two notches 70 formed therein forreceiving and holding a wire 72. Alignment mechanism 60 may be pivotedaway from vertical support wall 14 (as shown in FIGS. 2-4), or towardvertical support wall 14 (as shown in FIGS. 5-7). Alternatively, thealignment mechanism 60 may be constructed with no pivotal attachment. Inother words, the alignment mechanism 60 may be fixed in place and notallowed to pivot by, for example, eliminating the pivotal attachment andpivot pins 64.

FIG. 3 shows first fiber optic cable clamp 20 in an open position sothat fiber optic cable 100 may be received in curved portion 30 of firstfiber optic cable support 18. First fiber optic cable clamp 20 includesa body portion 40 pivotally attached to a foot portion 41 by a pivot 42.A protrusion 44 extends away from a surface of and is integral with bodyportion 40, and may comprise or contain a magnetic material. A stop 46also extends away body portion 40, and may comprise an elastomeric orresilient material such as rubber. First fiber optic cable clamp 20further includes a receiving portion 48 that may be made of a metallicmaterial or a magnetic material having a polarity opposite of thepolarity of magnetic protrusion 44. When clamp 20 is closed, stop 46 isreceived in the grooves of curved portion 30 of first fiber optic cablesupport 18 and fiber optic cable 100 is compressed between stop 46 andcurved portion 30, and magnetic protrusion 44 is received in receivingportion 48. The magnetic force created between magnetic protrusion 44and receiving portion 48 holds clamp 20 closed, and securely compressescable 100 between stop 46 and curved portion 30.

FIG. 4 shows second fiber optic cable clamp 24 in an open position sothat fiber optic cable 100 may be received in curved portion 34 ofsecond fiber optic cable support 22. Second fiber optic cable clamp 24includes a spring-biased cylindrical portion 50 having an extension 52integral with and extending away therefrom. A stop 54 is provided on aportion of extension 52 and may comprise an elastomeric or resilientmaterial such as rubber. Clamp 24 may be opened (as shown in FIG. 4) bylifting cylindrical portion 50 upward and rotating extension 52 awayfrom curved portion 34. Cylindrical portion 50 connects to the springhoused therein, and the spring force of the spring forces cylindricalportion 50 toward curved portion. Thus, to open clamp 24 a force must beexerted upon cylindrical portion 50 to overcome the spring force of thespring. When clamp 24 is closed, stop 54 is received in the grooves ofcurved portion 34 of second fiber optic cable support 22 and fiber opticcable 100 is compressed between stop 54 and curved portion 34. Thespring force created by the spring on cylindrical portion 50 holds clamp22 closed, and securely compresses cable 100 between stop 54 and curvedportion 34.

At least one magnetic clamp 20 is preferably used with apparatus 10since magnetic clamp 20 is easy to maneuver by a technician when holdingtwo portions of fiber optic cable 100. However, two magnetic clamps 20,two spring-biased clamps 24, or two similar types of clamps may be usedwith apparatus 10. Furthermore, magnetic clamp 20 may be switched withspring-biased clamp 24.

As further shown in FIG. 4, a pair of laser beams may be provided by alaser 104 located adjacent to the fiber optic cable apparatus 10 to etchlines in the glass optical fiber 102 that is exposed in the fiber opticcable 100 in a conventional etching process, as modified by apparatus 10of the present invention. Laser 104 is aligned to expose photosensitiveglass optical fiber 102 to patterned light emanating therefrom. Laser104 may be any conventional laser used to form a refractive-indexgrating within a fiber optic cable. FIG. 8 shows the resulting fiberoptic cable 100 and etch lines 106 formed in glass optical fiber 102 bya patterned laser beam 108.

FIGS. 5-7 show the tensioning apparatus 10 as a fiber optic cable 100 issecured in clamps 20,22 of apparatus 10, and with fiber optic cable 100in tension. As shown, a first portion of fiber optic cable 100 issecured in first clamp 20, that is, the first portion of cable 100 iscompressed between stop 46 of first clamp 20 and curved portion 30 offirst support 18. A second portion of fiber optic cable 100 is securedin second clamp 24, that is, the second portion of cable 100 iscompressed between stop 54 of second clamp 24 and curved portion 34 ofsecond support 22. Once fiber optic cable 100 is secured in clamps20,24, knob 26 is rotated, which in turn rotates cam 28, until cam 28 islocated at its position shown in FIG. 5. In this position, cam 28 stillcontacts leg portion 38 of second support 22, but leg portion 38 hasmoved upward in comparison to its location shown in FIG. 2. The movementof leg portion 38 causes second support 22 to rotate counterclockwiseabout pivot pin 36, causing a uniform tension to be applied to fiberoptic cable 100.

As shown in FIG. 5, second support 22 rotates counterclockwise at anangle θ. Angle θ is approximately between five to seven degrees, but mayvary depending upon the elastic properties of the fiber optic cablebeing tensioned and the weights of second support 22 and second clamp24. A uniform, repeatable tension is applied to cable 100 since secondclamp 24 and second support 22 have known weights, and the tensionapplied to cable 100 is due to gravity acting upon second clamp 24 andsecond support 22. Second clamp 24 and second support 22 create a momentarm that is opposed by a counter-moment created by cable 100 in auniform and repeatable manner.

The weights of second clamp 24 and/or second support 22 may be varied toprovide different tensions on different fiber optic cable types. Thismay be accomplished by, for example, constructing the second clampand/or second support 22 to have a desired weight or by adding removableweight(s) to clamp 24 and/or support 22. Another alternative to varyingthe amount of tension applied is to increase the length of the momentarm by, for example, constructing the second support 22 and/or secondclamp 24 to have a different length or by, for example, constructingsecond support 22 such that it has a threaded weight at the end that canbe screwed into and out of the second support 22 and thereby change thelength of the moment arm.

Apparatus 10 may be calibrated using a cable 100 having a knowndiffraction grating. To calibrate, untensioned cable 100 is provided inapparatus 10, and light having a known wavelength is injected into theuntensioned cable 100 with, for example, a laser. The wavelengthreflected by the known grating in cable 100 is measured by, for example,an OSA (optical spectrum analyzer). Alternatively, the wavelengthmeasurement of the untensioned cable may be made before loading cable100 into apparatus 10. Tension is then applied to the cable 100 usingthe apparatus 10 and another wavelength measurement made. By comparingthe wavelength shift (untensioned versus tensioned) and applyingconventional equations, the amount of tension applied by the apparatus10 may be precisely determined. In the same fashion, the repeatabilityof the tension applied by apparatus 10 to a series of cables 100 mayalso be assessed.

Although the method of tensioning fiber optic cable 100 has beendescribed above with reference to apparatus 10, a step-by-stepdescription of the method will be described with reference to FIG. 9.With first clamp 20 and second clamp 24 in their open positions and cam28 in its position shown in FIG. 2, a technician lays the first portionof fiber optic cable 100 in grooved curved portion 30 of first support18, and then closes first clamp 20, making sure that glass optical fiber102 is centered on apparatus 10. This secures a portion of cable 100 infirst support 18 (step 200). The second portion of cable 100 is thenplaced in grooved curved portion 34 of second support 22, cable 100 ispulled to eliminate slack, and second clamp 24 is rotated and closed.This secures another portion of cable 100 in second support 22 (step210). The technician then rotates knob 26 (and thus, cam 28) 180 degreesfrom its position shown in FIG. 2, or until cam 28 reaches the positionshown in FIG. 5. This permits second support 22 to rotate at angle θ,uniformly tensioning fiber optic cable 100 (step 220). The technicianmay then energize laser 104 and begin the process for etching arefractive-index grating in glass optical fiber 102 of cable 100 (step230). Once the grating is etched, cable 100 may be removed fromapparatus 10 (step 240) by reversing the previous method steps. Ifanother grating is to be written, the method is repeated at step 250, ifnot, the method is terminated at step 260.

It will be apparent to those skilled in the art that variousmodifications and variations can be made in the fiber optic cabletensioning apparatus and method of the present invention and inconstruction of the apparatus and method without departing from thescope or spirit of the invention. As an example, although stainlesssteel is the preferred material for the components of the apparatus ofthe present invention, other similarly stable materials may be used.Furthermore, as described previously, the tension applied to cable 100may be varied to create different diffraction gratings with the presentinvention. Alternatively, the tension may be held uniform with thepresent invention, and the wavelength of the laser beams etching thediffraction grating may be varied to create different diffractiongratings.

Other embodiments of the invention will be apparent to those skilled inthe art from consideration of the specification and practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with a true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A method for tensioning and positioning a fiberoptic cable, comprising: securing a first portion of the fiber opticcable to a first support; securing a second portion of the fiber opticcable to a second support, said second support having a curved portion,a radius associated with said curved portion exceeding a minimum bendradius of said fiber optic cable ; and creating a gravity-assistedmoment arm with the second support to uniformly and repeatably tensionand position the fiber optic cable between the first and secondsupports.
 2. A method for tensioning and positioning a fiber optic cableas recited in claim 1, wherein the gravity-assisted moment arm iscreated by rotating a cam contacting the second support to rotate thesecond support due to its weight.
 3. A method for tensioning andpositioning a fiber optic cable as recited in claim 2, furthercomprising, prior to securing the first portion of the fiber opticcable, rotating the cam to rotate the second support in a directionopposite to the direction that uniformly and repeatable tensions andpositions the fiber optic cable.
 4. A method for tensioning andpositioning a fiber optic cable as recited in claim 1, wherein the firstportion of the fiber optic cable is secured to the first support with afirst clamp.
 5. A method for tensioning and positioning a fiber opticcable as recited in claim 1, wherein the second portion of the fiberoptic cable is secured to the second support with a second clamp.
 6. Amethod for tensioning and positioning a fiber optic cable as recited inclaim 2, wherein the second support comprises a rotatable body portionintegrally connected to a leg portion, the leg portion contacting thecam to rotate the second support.
 7. A method for tensioning andpositioning a fiber optic cable as recited in claim 1, furthercomprising aligning a glass optical fiber portion of the fiber opticcable with an alignment mechanism provided between the first and secondsupports.
 8. A method for forming a refractive-index grating in a fiberoptic cable, comprising: securing a first portion of the fiber opticcable to a first support; securing a second portion of the fiber opticcable to a second support, said second portion of said fiber optic cablehaving a curved portion, a radius associated with said curved portionexceeding a bend radius of said fiber optic fiber; creating agravity-assisted moment arm with the second support to uniformly andrepeatably tension and position the fiber optic cable between the firstand second supports; and etching grating lines in the fiber optic cableafter the fiber optic cable has been uniformly and repeatably tensionedand positioned.
 9. A method for forming a refractive-index grating in afiber optic cable as recited in claim 8, wherein the gravity-assistedmoment arm is created by rotating a cam contacting the second support torotate the second support due to its weight.
 10. A method for forming arefractive-index grating in a fiber optic cable as recited in claim 9,further comprising, prior to securing the first portion of the fiberoptic cable, rotating the cam to rotate the second support in adirection opposite to the direction that uniformly and repeatabletensions and positions the fiber optic cable.
 11. A method for forming arefractive-index grating in a fiber optic cable as recited in claim 8,wherein the first portion of the fiber optic cable is secured to thefirst support with a first clamp.
 12. A method for forming arefractive-index grating in a fiber optic cable as recited in claim 8,wherein the second portion of the fiber optic cable is secured to thesecond support with a second clamp.
 13. A method for forming arefractive-index grating in a fiber optic cable as recited in claim 9,wherein the second support comprises a rotatable body portion integrallyconnected to a leg portion, the leg portion contacting the cam to rotatethe second support.
 14. A method for forming a refractive-index gratingin a fiber optic cable as recited in claim 8, further comprisingaligning a glass optical fiber portion of the fiber optic cable with analignment mechanism provided between the first and second supports. 15.A method for calibrating a fiber optic cable tensioning and positioningapparatus having a first support and a second support rotatable relativeto the first support, comprising: securing a first portion of the fiberoptic cable to the first support; securing a second portion of the fiberoptic cable to the second support, said second support having a curvedportion, a radius associated with said curved portion exceeding a bendradius of said fiber optic cable; measuring a diffraction gratingprovided in the untensioned fiber optic cable; creating agravity-assisted moment arm with the second support to uniformly tensionthe fiber optic cable between the first and second supports; measuringthe diffraction grating provided in the uniformly tensioned fiber opticcable; and comparing the measured diffraction grating of the untensionedfiber optic cable to the measured diffraction grating of the tensionedfiber optic cable to calculate the tension applied to the fiber opticcable.